<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="../../js/vue.js"></script>
</head>
<body>
<div id="div_id">
    <h2>{{l_name+" "+f_name}}</h2>
    <h2>{{l_name}} {{f_name}}</h2>
    <h2>{{getFullName()}}</h2>
    <h2>{{fullName}}</h2>
</div>

<script>
    const app=new Vue({
        el:"#div_id",
        data:{
            f_name:'J',
            l_name:'L'
        },
        methods:{
          getFullName: function(){
              return this.l_name+" "+this.f_name;
          }
        },
        computed:{
            fullName:function (){
                return this.l_name+" "+this.f_name
            }
        }
    })
</script>
</body>
</html>